The alignment of a vehicle's wheel plane WP relative to the path traveled by the vehicle affects not only the handling of the vehicle but also affects the wear on the tires. As used here, alignment refers to camber, toe, and thrust. Referring to FIG. 1, camber is the angle between the wheel plane WP and a vertical axis VA of the vehicle 60. Positive camber (+C) refers to an angle where the top of the wheel 50 is farther away from the center of vehicle 60 than the bottom of the wheel 50. Negative camber (−C) refers to an angle where the bottom of the wheel 50 is farther away from center of the vehicle 60 than the top. Generally speaking, camber changes of even a fourth of one degree can impact tire wear. Abnormal tire wear has been observed in certain applications with even smaller changes in camber angle. Free rolling (non-driven) tires in low wear rate applications are especially sensitive to camber and thus particularly prone to developing abnormal wear if the camber angle is unfavorable.
Referring to FIG. 2, toe is the angle the wheel plane WP makes with a centerline along the longitudinal axis LA of the vehicle 60. Positive toe (+T), also referred to as toe in, is a condition where the front of the wheel 50 or the wheel plane WP is pointing in or towards the center line of the vehicle 60. Negative toe (−T), also referred to as toe out, is a condition where the front of the wheel 50 or wheel plane WP points out or away from the center line of the vehicle 60. Thrust is the resulting direction of travel (FDT) of an axle as opposed to the direction that might be expected from the orientation of wheel planes WP of the wheels on the axle. Generally speaking, toe changes of even one-tenth of a degree can have an impact on tire wear.
The typical trailer axle is made by welding a pair of spindle forgings onto a piece of axle tubing then machining the precision surfaces of both spindles simultaneously in a lathe process. The resulting axle is near perfectly straight—i.e., each spindle axis possesses zero camber and zero toe. When a typical axle is installed under a vehicle (used herein to refer to both motorized vehicles as well as trailers) and placed into normal operation under typical loading conditions, the camber does not remain at zero. The axle under load, although quite rigid, flexes. The flexing of the axle occurs because the suspension is attached to the axle at load transfer points which are significantly inboard of the ends of the axle, but the tires support the weight of the vehicle by means of attachment points which are relatively near the outboard ends of the axle. As a result of this geometry, the weight of the vehicle imposes a bending moment on the axle which in turn causes upward deflection of the ends of the axle resulting in the tires presenting a slight negative camber. As the load increases, the more negative the camber becomes. At the typical maximum legal tandem axle load in the United States, it would not be unusual for the wheel camber angle to reach approximately 0.5 degrees. The contribution of tire alignment to tire wear can be particularly problematic with vehicles used for transporting heavy loads.
Once the weight is removed, the axle may recover and again affect the alignment of the wheels. Because of factors such as the additional costs and amount of material that would be required, increasing the stiffness of the axle to resolve camber issues may not be practical.
Even with the same amount of camber on each axle spindle, axle camber affects the tires differently depending on their individual wheel end position on the vehicle because most road surfaces (RS) are not flat transversely (orthogonal to the normal travel direction) across the road. The road surface is either crowned or sloped (by about 1.5% on average) so that water will evacuate from the road surface. Trucks, in North America and other countries using the right side of the road for forward traffic, generally operate in the right most lane, which is usually sloped very slightly to the right. This means that as vehicle is traveling on the road way, there is a gravitational force pulling the vehicle to the right. This force is resisted through the tire contact patch, and the tire transmits this force to the axle by transmitting the required force opposite of the direction of pull through its interface with its wheel. The result is that as the tire rolls down the highway, the contact patch shifts leftward with respect to the wheel plane WP. At full load and at normal pressure on a typical New Generation Wide Base Single tire (NGWBS tire), this shift has an effect on tire shoulder wear that is roughly the equivalent of a 0.2 degree shift in wheel camber. This means that, although the left and the right wheel may each measure approximately −0.5 degree of camber, when the shift effect is considered, the effective camber angle on the left side tires is approximately −0.7 degree, and the effective camber angle on the right side tires is approximately −0.3 degree. As a consequence of this phenomenon, tires on the driver side left of the vehicle usually experience worse inboard shoulder wear than tires on the driver side right of the vehicle.
When a typical tandem axle vehicle (tractor or trailer) turns, the dynamics of the vehicle favor lateral grip by the forward axle tires. As a result, the pivot point of the vehicle shifts toward the forward axle tires, and the rear axle tires will tend to have greater slip laterally as the vehicle negotiates a turn. For this reason, the rear tires on a tandem axle pair receive more scrub and have a faster wear rate than the tires on the forward axle. Scrub tends to arrest the development of irregular wear and thus the rear tires usually are less affected by the camber issue than are the tires on the forward axle.
As a consequence, irregular tire wear is usually worst on the inboard surface of the LF tire. Next worst is the LR tire. The RF tire comes next but is sometimes similar in severity to the LR. The most even wear usually is found on the RR tire depending upon the particular application, load, and routes normally traveled. It should be obvious that in countries such as Australia, where drivers drive on the left side of the road instead of the right side, such observations would be reversed.
Therefore, a need exists for improved methods and apparatus for adjusting or correcting axle alignment and, more particularly, for allowing adjustment to camber, toe, and thrust. A system that allows for select adjustments—i.e. adjustments by discrete, predetermined amounts would be useful. Such a system that allows for a wide range in variation of adjustment and for the indexing of such adjustments would also be useful. Additional usefulness would be provided by a system that allows for adjustment of the alignment of an axle using hardware that can be used for the left or right sides of the vehicle.